Helium droplet spectroscopy is currently established as one of the leading experimental techniques for the production and study of novel clusters and complexes. The success of the approach is largely due to two factors: the remarkable versatility of the approach as an assembly technique to produce novel species, and the method's facilitation of spectroscopic interrogation of the species produced. The low droplet temperature (e.g., 0.37 K) and high mobility of dopants inside the droplet result in efficient coagulation of captured impurities into clusters and complexes. Because the helium “solvent” interacts so weakly with dopants placed in the droplets, coagulation (rather than solvent-separation) of the dopants occurs quickly for virtually any combination of atoms and molecules chosen. Many species formed in helium droplets would be either difficult or impossible to form by other methods.
The success of the helium droplet spectroscopy approach is not solely due to versatility. Its success can also be attributed to the method's ability to facilitate the spectroscopic interrogation of different species. Spectroscopic measurements, ranging from the microwave to the ultraviolet, are possible because the helium droplet itself is transparent to photons with energies less than 20 eV. Additionally, the low temperature in the droplet tends to greatly simplify the spectra obtained. As mentioned above, the interaction between the assembled species and the surrounding helium is extremely weak. Consequently, the perturbations to the species are typically minimal, and the gas phase Hamiltonian continues to be applicable. Notably, the superfluid nature of the helium solvent permits the rotation of the solvated structure (albeit with modified rotational constants), thus preserving the rotational fine structure in the spectra.
In contrast to the aforementioned advantages, one significant disadvantage to potential practitioners of helium droplet spectrometry is the costs associated with the equipment required to setup the experiments. More specifically, the laser system used to excite transitions in the species under study, and also the detector necessary to monitor the photon-induced depletion of the droplet beam, can be expensive to implement. Recent advances in laser technology such as the increased availability and performance of quantum cascade lasers (QCLs), for example, are creating more cost-effective options for laser sources. In regards to beam detection, most users utilize either a bolometer or a quadrupole mass spectrometer (QMS). The bolometer is the less expensive and more sensitive of the two options, but it still uses a liquid helium cryostat to ensure sufficiently sensitive detection. While the QMS does not require a liquid helium supply, it is typically more expensive to purchase.
The paper titled “Infrared spectroscopy of helium nanodroplets: novel methods for physics and chemistry” by M. Y. Choi et al. discusses the typical prior art arrangements of a helium droplet mass spectrometer arrangement 100 and helium droplet bolometer arrangement 200 as illustrated respectively in FIGS. 1 and 2 infra. Referring to FIG. 1, the schematic diagram shows the helium droplet mass spectrometer arrangement 100 including a helium droplet source chamber 110, pick-up cell chamber 120 and an off-axis mass spectrometer 130 that receives an infrared laser light 140 directed at the droplet beam 111 along the droplet beam path 121. FIG. 2 shows helium droplet bolometer arrangement 200 including a helium droplet source chamber 210, pick-up cell chamber 220, laser multipass chamber 230 and a bolometer chamber 240. The laser multipass chamber 230 is used to cross the droplet beam 211 with multiple laser 250 passes.
Known techniques accomplish intentional doping of the droplets with the species of interest using a ‘pick-up’ technique which involves passing the beam 111/211 through the pick-up cell chamber 120/220, that is maintained at a pressure sufficient to permit the capture of the desired number of gas-phase atoms or molecules. The pick-up cells, shown schematically in FIGS. 1 and 2 as being within the pick-up cell chamber 120/220, may take on many different forms depending upon the dopant molecules or atoms of interest. Due to the optical transparency of the droplets, a wide range of possibilities for the cells exist, including simple gas cells and high-temperature ovens designed to evaporate highly refractive materials.
The infrared spectrum of a solvated molecule can therefore be obtained by recording the frequency dependence of the laser-induced attenuation of the helium beam. As shown in FIGS. 1 and 2, this recording may be accomplished using either a mass spectrometer 130 or bolometer chamber 240 to measure the helium beam flux, as would be appreciated by those skilled in the art.
Given the cost and complexity of current detectors, there exists an unmet need in the art for an effective, yet relatively inexpensive, droplet beam detector for use in such spectroscopy systems and methods.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding description constitutes prior art against the present invention.